1. Field of the Invention
A grain-oriented electrical steel sheet is used as a core material for electric devices such as a transformer and this grain-oriented electrical steel sheet should have superior magnetic properties such as exciting characteristics and core loss characteristics. The magnetic flux density B.sub.8 at a magnetic field intensity of 800 A/m is generally used as the numerical value representing the exciting characteristics, and the core loss W.sub.17/50 per kg observed when the sheet is magnetized to 1.7 Tesla (T) at a frequency of 50 Hz is used as the numerical value representing the core loss characteristics. The magnetic flux density is a factor having the most influence on the core loss characteristics, and in general, the higher the magnetic flux density, the better the core loss characteristics. Nevertheless, an increase of the magnetic flux density generally results in an increase of the size of secondary recrystallized grains, and sometimes the core loss characteristics are lowered. In contrast, the core loss characteristics can be improved, regardless of the size of the secondary recrystallized grains, by controlling the magnetic domain.
This grain-oriented electrical steel sheet is prepared by a secondary recrystallization at the final finish annealing step, to develop the Goss structure in which a {110} plane is formed on the surface of the steel sheet and a &lt;001&gt; axis is produced in the rolling direction.
To obtain good magnetic characteristics, the easy magnetization axis &lt;001&gt; must be arranged precisely in line with the rolling direction.
Typical instances of this process for the preparation of a grain-oriented electrical steel sheet having a high magnetic flux density are disclosed in Japanese Examined Patent Publication No. 40-15644 to Satoru Taguchi et al, and Japanese Examined Patent Publication No. 51-13469 to Takuichi Imanaka et al. In the former process, MnS and AlN are used as the main inhibitor, and in the latter process, MnS, MnSe and Sb are used as the main inhibitor. Therefore, according to the presently available technique, the size, shape and dispersion state of precipitates acting as the inhibitor must be controlled. For example, in connection with the MnS, a method is adopted in which MnS is once solid-dissolved at the step of heating a slab before hot rolling and MnS is precipitated at the hot rolling step. A temperature of about 1400.degree. C. is necessary for completely solid-dissolving MnS in an amount necessary for the secondary recrystallization, and this temperature is higher by more than 200.degree. C. than the slab-heating temperature adopted for a usual steel. This high-temperature slab-heating treatment has the following disadvantages.
(1) A high-temperature slab-heating furnace exclusively used for the production of a grain-oriented electrical steel sheet is necessary.
(2) The energy unit of the heating furnace is high.
(3) The amount of melted scale is increased, and the operation efficiency is reduced by a drain-off of the slag.
These disadvantages will be overcome if the slab-heating temperature is lowered to the level adopted for a usual steel, but this means that the amount of MnS effective as the inhibitor must be reduced or MnS not used at all, which results in an unstable secondary recrystallization. Accordingly, to realize a low-temperature heating of the slab, the inhibitor must be intensified by precipitates other than MnS, by one means or another and the growth of normal grains at the finish annealing properly controlled. As such an inhibitor, sulfides, nitrides, oxides, and grain boundary-precipitated elements are considered to be effective, and for example, the following known techniques can be mentioned.
Japanese Examined Patent Publication No. 54-24685 discloses a method in which the slab-heating temperature is adjusted to 1050.degree. to 1350.degree. C. by incorporating into a steel a grain boundary-segmented element such as As, Bi, Sn or Sb, and Japanese Unexamined Patent Publication No. 52-24116 discloses a method in which the slab-heating temperature is adjusted to 1100.degree. to 1260.degree. C. by incorporating a nitride-forming element such as Al, Zr, Ti, B, Nb, Ta, V, Cr or Mo. Furthermore, Japanese Unexamined Patent Publication No. 57-158322 discloses a technique of lowering the slab-heating temperature by reducing the Mn content and adjusting the Mn/S ratio to less than 2.5, and stabilizing the secondary recrystallization by an addition of Cu. Separately, a technique has been proposed of improving the metal structure in combination with the intensification of the inhibitor. Namely, Japanese Unexamined Patent Publication No. 57-89433 discloses a method in which a low-temperature heating of a slab at 1100.degree. to 1250.degree. C. is realized by incorporating an element such as S, Se, Sb, Bi, Pb, Sn or B in addition to Mn, and simultaneously, controlling the columnar crystal ratio in the slab and the reduction ratio at the second cold rolling step. Furthermore, Japanese Unexamined Patent Publication No. 59-190324 proposes a technique of stabilizing the secondary recrystallization by incorporating S and Se, forming an inhibitor mainly by Al, B and nitrogen, and carrying out a pulse annealing at the primary recrystallization annealing conducted after cold rolling.
The present inventors previously proposed a technique of realizing a low-temperature heating of a slab by controlling the Mn content to 0.08 to 0.45% and the S content to less than 0.007%, in Japanese Unexamined Patent Publication No. 59-56522. According to this method, the problem of an insufficient linear secondary recrystallization in a product, which is due to a coarsening of the crystal grains of the slab during the high-temperature heating of the slab, can be solved.
The primary object of this low-temperature slab-heating method is to reduce the manufacturing cost, but the method cannot be industrialized unless good magnetic properties can be stably obtained. If the slab-heating temperature is lowered, changes at the hot rolling step, such as lowering of the hot rolling, should naturally be made, but the continuous production process comprising a low-temperature heating of a slab, including the hot rolling step, has not been investigated.
In the conventional high-temperature slab-heating (for example, at a temperature higher than 1300.degree. C.), the main roles of hot rolling are the following three rolls, that is, (1) a division of coarse crystal grains by recrystallization, (2) a precipitation of fine MnS and AlN or control of the precipitation, and (3) a formation of {110}&lt;001&gt; oriented grains by shear deformation. In the low-temperature heating of the slab, the role (1) is not necessary, and the role (2) is sufficiently exerted if an appropriate microstructure is produced after decarburization annealing, as taught by in Japanese Patent Application No. 1-1778, and therefore, a control of the precipitates in the hot-rolled sheet is not necessary. Accordingly, the restrictions of the conventional hot rolling method are moderated in the low-temperature heating of the slab.
Therefore, the inventors examined the hot rolling method in which, to control the secondary recrystallization, the microstructure of a hot-rolled steel sheet is rationalized to a high level not attainable by the conventional high-temperature slab-heating method. For example, in connection with metal-physical phenomena after the final pass of hot rolling, a precipitation of fine MnS and AlN or control of the precipitation is a most important control item in the conventional method, and other phenomena are not taken into consideration.
The inventors noted the recrystallization phenomenon after the final pass of the finish hot rolling, not taken into consideration in the conventional techniques, and examined the hot rolling method for obtaining a product having good and stable magnetic properties by utilizing this phenomenon for controlling the microstructure of a hot-rolled steel sheet in the preparation process in which the low-temperature heating of the slab is carried out as the premise step and the final high-reduction cold rolling is carried out at a reduction ratio of at least 80%.
In connection with a hot rolling of a grain-oriented electrical steel sheet, as the means for preventing an insufficient secondary recrystallization (formation of linear fine grains continuous in the rolling direction) caused by a coarsening growth of the crystal grains of the slab by a high-temperature heating of the slab, a method has been proposed in which coarse crystal grains are divided by recrystallization high-reduction rolling conducted at a hot rolling temperature of 960.degree. to 1190.degree. C. and a reduction ratio of at least 30% per pass (Japanese Examined Patent Publication No. 60-37172), and the formation of linear fine grains can be moderated by this method, but this method requires the high-temperature heating of the slab to be carried out as the premise operation.
In the low-temperature heating of the slab (lower than 1280.degree. C.), the above-mentioned coarsening of crystal grains caused by the high-temperature heating of the slab is not caused, and therefore, the recrystallization high-reduction rolling for a division of coarse crystal grains is not necessary.
In connection with the preparation process using MnS, MnSe or Sb as the inhibitor, a method has been proposed in which hot rolling is continuously carried out at a reduction ratio of at least 10% at a hot rolling temperature of 950.degree. to 1200.degree. C., and then the hot-rolled product is cooled at a cooling rate of at least 3.degree. C./sec to finely and uniformly precipitate MnS, MnSe or the like, whereby the magnetic properties are improved (Japanese Unexamined Patent Publication No. 51-20716). Furthermore, a method has been proposed in which the advance of the recrystallization is restrained by carrying out hot rolling at a low temperature, and the magnetic properties are improved by preventing a reduction of the {110}&lt;001&gt; oriented grains at the subsequent recrystallization (Japanese Examined Patent Publication No. 59-32526 and Japanese Examined Patent Publication No. 59-35415). Even in these methods, the preparation process in which the low-temperature heating of a slab is carried out as the premise operation and the high-reduction final cold rolling is carried out at a reduction ratio of at least 80% is not examined. Still further, in connection with hot rolling of a silicon steel slab having a carbon content lower than 0.02% by weight, a method has been proposed in which a low-temperature high reduction hot rolling, which results in an accumulation of strain in the hot-rolled sheet, is carried out, and at the subsequent annealing of the hot-rolled sheet, coarse crystal grains peculiarly formed in a steel having an especially low carbon content are divided by the recrystallization (Japanese Examined Publication No. 59-34212). But, according to this method, it is difficult to obtain good stable magnetic properties.